Alzheimer's disease (AD) is the most prevalent form of dementia. It is a neurodegenerative disorder, clinically characterized by progressive loss of memory and general cognitive function, and pathologically characterized by the deposition of extracellular proteinaceous plaques in the cortical and associative brain regions of sufferers. These plaques mainly comprise fibrillar aggregates of beta-amyloid peptide (Aβ). Aβ is formed from amyloid precursor protein (APP). APP is a ubiquitous membrane-spanning (type 1) glycoprotein, of which three major isoforms (APP695, APP751, and APP770) are known, that undergoes a variety of proteolytic processing events (Selkoe, 1998, Trends Cell Biol. 8:447-453).
Generation of Aβ from APP occurs via separate intracellular proteolytic events involving the enzymes beta-secretase and γ-secretase. Beta-secretase first cleaves APP within the extracellular domain to create soluble APP-beta and beta-CTF (C-terminal fragment), which is then further processed by γ-secretase to release Aβ and γ-CTF. Given that γ-secretase cleaves beta-CTF, beta-CTF has widely been used to monitor γ-secretase activity in cell based and in vitro assays. The cleavage site of APP by γ-secretase appears to be situated within a transmembrane domain, and variability in the site of γ-secretase mediated proteolysis results in Aβ of varying chain lengths comprising heterogeneous C-termini, e.g. Aβ (1-38, “Aβ38”), Aβ (1-40, “Aβ40”) and Aβ (1-42, “Aβ42”). After secretion into the extracellular medium, the initially-soluble Aβ forms aggregate, ultimately resulting in the insoluble deposits and dense neuritic plaques which are the pathological characteristics of AD. Aβ42 is more prone to aggregation than Aβ40 and is the major component of amyloid plaque (Jarrett, et al., 1993, Biochemistry 32:4693-4697; Kuo, et al., 1996, J. Biol. Chem. 271:4077-4081).
Alternatively, APP can be sequentially cleaved by alpha-secretase and γ-secretase to produce soluble APP-alpha, P3 and γ-CTF. Alpha-secretase cleavage precludes the formation of Aβ peptides.
Various interventions in the plaque-forming process have been proposed as therapeutic treatments for AD (see, e.g., Hardy and Selkoe, 2002, Science 297:353-356). One such method of treatment that has been proposed is that of blocking or attenuating the production of Aβ, for example, by inhibition of beta- or γ-secretase. Other proposed methods of treatment include administering a compound(s) which blocks the aggregation of Aβ, or administering an antibody which selectively binds to Aβ. Activation of α-secretase is also an appealing strategy for the development of AD therapy, in that increased alpha-secretase cleavage might lend to lessened Aβ generation.
γ-secretase is a macromolecular aspartyl protease composed of at least four proteins: presenilin (PS), nicastrin (NCT), PEN-2 and APH-1 (De Strooper, 2003, Neuron 38:9-12). Recently, CD147 and TMP21 have been found to be associated with the γ-secretase complex (Chen, et al., 2006, Nature 440:1208-1212; Zhou et al., 2005, Proc. Natl. Acad. Sci. USA, 102:7499-7504). Among these known components, PS is believed to contain the active site of γ-secretase (Esler et al., 2000, Nat. Cell. Biol., 2:428:434; Li et al., 2000, Nature 405:689-694; Wolfe et al., 1999, Nature 398:513-517). Considerable effort has been made to understand the process of γ-secretase substrate recognition and its catalytic machinery. A PS-dependent protease can process any single-pass transmembrane (TM) protein regardless of its primary sequence as long as the TM protein extracellular domain is smaller than 300 amino acids. Moreover, the size of the extracellular domain appears to determine the efficiency of substrate cleavage (Struhl and Adachi, 2000, Mol. Cell. 6:625-636).
The sequential cleavage of APP by two proteases (beta- or alpha-secretase followed γ-secretase) is analogous to a recently defined signaling paradigm, known as regulated intramembrane proteolysis (RIP) (Brown et al., 2000, Cell 100:391-398). RIP generally requires two proteolytic steps to initiate its signaling cascade, whereby the second intramembrane cleavage is dependent on the first cleavage. Indeed, Notch, a type I transmembrane protein employs RIP and is a substrate for γ-secretase cleavage. Activation of Notch (which is γ-secretase dependent) has been implicated in cancer development. As such, inhibitors of γ-secretase activity might not only have implications in the treatment of AD, but may also have benefit in treatment of all diseases in which γ-secretase plays a role.
Cancer also affects a significant number of people. It is currently believed that the Notch signaling pathway is implicated in cancer biology. The Notch signaling pathway involves cell-cell communication, and aberrant Notch signaling has been observed in cancer cells. Such aberrant Notch signaling has been linked to tumor formation. γ-Secretase inhibitors have been found to prevent the generation of the active domain of Notch molecules, thereby suppressing Notch signaling.
There is a need in the art for additional treatments for cancer and neurodegenerative diseases, such as Alzheimer's disease.